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Dr. W. M. Bayliss. 



[June 30, 



As will be seen later, the close correspondence between the osmotic 

 pressure found and that of the dye if un dissociated must be due to the 

 chemical nature of this particular dye as a disodiuni salt of a dibasic acid. 

 Other dyes of a similar^ constitution, but of different sodium content, such as 

 Chicago blue, do not show this property. I regard it as a somewhat 

 unfortunate accident that Congo red was chosen as the object of the first 

 investigation. Attention was thereby diverted from the more essential facts. 



As it will frequently be necessary to refer to the osmotic pressure as it 

 would be shown by a body present in solution in undissociated single 

 molecules, I propose, for convenience, to speak of it as the " molecular " 

 osmotic pressure, although of course the expression is not strictly correct. 



Hydrolytic Dissociation. 



It is fortunate, as an initial simplification of the problem, that no trace of 

 hydrolytic dissociation can be detected in solutions of Congo red. When 

 such solutions are separated from water by parchment-paper, no free alkali 

 diffuses out, such as happens, for example, from solutions of sodium oleate. 

 In this result I find myself in agreement with other observers.* The acid 

 of Congo red, in fact, behaves as a strong one, no doubt owing to the two 

 sulphonic acid groups contained in its molecule. Its solutions attack metallic 

 zinc. In its use as an indicator it is well known that the second sodium 

 atom can only be displaced by strong mineral acids. It is evident that the 

 NH 2 groups of the naphthylamine residues are practically neutralised by the 

 sulphonic acids. The basic properties of this amino-acid are so weak as to 

 be negligible. I have been unable, indeed, to find any evidence that it forms 

 salts even with hydrochloric acid. 



It is interesting to note incidentally that even sodium caseinogenate 

 appears to be hydrolytically dissociated only to a minute degree. Eoaff 

 makes the same statement with respect to the sodium salts of the serum 

 proteins. Hardy} also found the hydrolysis of the sodium salt of globulin 

 to be very slight. 



Electrolytic Dissociation. 

 Although, since my former experiments were made, measurements of the 

 conductivity of Congo-red solutions have been published,! I thought it best 

 to determine that of the particular sample of dye used for the experiments 



* Pelet and Wild, 'Kolloid. Zeits.,' 1908, vol. 3, p. 174. 



t ' Quart. Journ. Exp. Phys.,' 1910, vol. 3, p. 175. 



1 'Journ. Physiol.,' 1905, vol. 33, p. 276. 



§ 'Die Theorie d. Farbeprozesses,' von L. Pelet- Joli vet. Dresden, 1910, p. 27. 



